In certain applications of set screws, overtightening is likely to result in damage to one or more of the parts involved. For example, in pipe couplings in which the set screw extends through a collar and seats against a pipe, excessive torque on the set screw may deform or crush the pipe wall. In some installations this will damage the joint and cause leakage. This is a common problem in many different kinds of pipe installations, such as those used in water works in which large metal pipes are joined to various fittings by flanged couplings.
In order to obtain the desired holding force without causing damage to the joint, the practice has been to tighten the set screw to a specified torque. This has been done by using torque limiting set screws of the type having a driving head which will shear off at a predetermined torque. In order to enhance the holding force, the set screw is commonly provided with a cone point or a cup point to indent the surface of the pipe and hold it against movement. The use of a torque limiting set screw as described is not always effective to prevent damage to the joint because over-tightening may still occur under some conditions. Accordingly, there is a need for a set screw which cannot be over-tightened to the point that it damages the part on which it is seated.
Various types of torque limiting fasteners are known in the prior art. In British Patent No. 898,026 (1959) a torque limiting bolt is disclosed comprising a threaded shank and first and second driving heads at one end of the shank. The heads are joined by a torque limiting neck which has a torsional strength less than the other parts of the bolt so that the outer head twists off when a predetermined torque is applied to it. The other head remains for unscrewing the bolt. The Reiland U.S. Pat. No. 3,812,757 discloses a torque limiting fastener with a threaded shank and primary and secondary driving heads. The primary driving head is formed as a socket and the wall of the socket is dimensioned so that it has a predetermined breaking strength. In the Schuster et al U.S. Pat. No. 3,498,174 primary and secondary driving heads are provided and the neck between the heads is the torque limiting structure.
The prior art discloses pipe couplings wherein a retaining collar is mounted on a pipe end by set screws extending through the collar and seated on the pipe. The pipe is joined to a flanged fitting by an adaptor body which mounts on the flange of the fitting. The collar is then bolted to the adaptor body with suitable gasketing to provide a fluid tight joint.
In the Piatek et al U.S. Pat. No. 4,165,892 a pipe coupling is disclosed in which a bushing receives a pipe end which is clamped in position by means of a locking pin. The locking pin comprises a stem with a camming head on the inner end and a pipe engaging point on the head. The locking pin extends through the wall of the bushing and is rotatable therein by a tool engaging head on the outer end. When the pin is rotated from its unlocked position, the camming head engages the bushing and is thrust inwardly toward the pipe. The pipe engaging point bites into and grips the pipe to hold it against movement.
A general object of this invention is to provide a set screw which overcomes certain disadvantages of the prior art.